A computer model simulating the flow in fuel injection systems has been used in order to investigate the fuel injection processes in an in-line pump-based fuel injection system for direct-injection diesel engines. The model is one-dimensional and it is based on the mass and momentum conservation equations for the simulation of the fuel flow and on the equilibrium of forces for the simulation of the mechanical movements of the valves present in the system. The fuel injection system tested comprised an in-line pump whose characteristics were examined by using as input the measured line pressure signal and by modeling the pump operation itself as well as the fuel flow through single- and two-stage injectors.For the validation of the model, extensive comparison with experimental data has been performed for a wide range of pump operating conditions. Measured quantities include the pressure inside the pump, the line pressure (at both the pump end and nozzle end of the high pressure pipe), the needle lift, the fuel injection rate, the total fuel injection quantity and the injection timing.Following successful validation of the model, parametric studies were performed focusing on the effect of various critical geometric parameters of the system on the nozzle exit characteristics, which could allow improved design of existing FIE systems in order to meet future target performance levels.